Mycelium is the vegetative part of a fungus or fungus-like bacterial colony, consisting of a mass of branching, thread-like hyphae. The mass of hyphae is sometimes called shiro, especially within the fairy ring fungi. Fungal colonies composed of mycelium are found in and on soil and many other substrates. A typical single spore germinates into a homokaryotic mycelium, which cannot reproduce sexually; when two compatible homokaryotic mycelia join and form a dikaryotic mycelium, that mycelium may form fruiting bodies such as mushrooms. A mycelium may be minute, forming a colony that is too small to see, or it may be extensive:

Is this the largest organism in the world? This 2,400-acre [970-hectare] site in eastern Oregon had a contiguous growth of mycelium before logging roads cut through it. Estimated at 1,665 football fields in size and 2,200 years old, this one fungus has killed the forest above it several times over, and in so doing has built deeper soil layers that allow the growth of ever-larger stands of trees. Mushroom-forming forest fungi are unique in that their mycelial mats can achieve such massive proportions.

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One of the primary roles of fungi in an ecosystem is to decompose organic compounds. Petroleum products and some pesticides (typical soil contaminants) are organic molecules (i.e., they are built on a carbon structure), and thereby present a potential carbon source for fungi. Hence, fungi have the potential to eradicate such pollutants from their environment unless the chemicals prove toxic to the fungus. This biological degradation is a process known as bioremediation.

Mycelial mats have been suggested (see Paul Stamets) as having potential as biological filters, removing chemicals and microorganisms from soil and water. The use of fungal mycelium to accomplish this has been termed mycofiltration.

When spread on logging roads, mycelium can act as a binder, holding new soil in place and preventing washouts until woody plants can be established.

Since 2007, a company called Ecovative Design has been developing alternatives to polystyrene and plastic packaging by growing mycelium in agricultural waste. The two ingredients are mixed together and placed into a mold for 3–5 days to grow into a durable material. Depending on the strain of mycelium used, they make many different varieties of the material including water absorbent, flame retardant, and dielectric.[2]